Creating a character separator for a textbox

Question

I am doing some more preparation for the university and am sort of stuck now. I am trying to make hexadecimal values in a textbox be outlined using dashes('-') while I am typing thus it should be done dynamically.

To be a bit more precise here are some pictures of what I am trying to do:

If I am typing hexes like this:

I want them to get separator dashes while I am typing them:

So far I have tried doing it with a for loop and I think that it should have worked out but it doesn't print out my values but rather just the dashes sans the text.

Here is the code bit:

   int i = 0;
        private void inputBox_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (hexRadioButton.Checked)
            {
                if (!char.IsControl(e.KeyChar))
                {
                    if (i < 2)
                    {
                        i++;
                        e.Handled = true;
                    }

                    else
                    {
                        i = 0;
                        inputBox.AppendText("-");
                        e.Handled = true;
                    }
                }
            }
    }

Answer 1

One approach would be to validate the whole string any time the text changes. This will account for pasting text as well as typing. It's a little brutish, but for a relatively short string it should do the job:

private void inputBox_TextChanged(object sender, EventArgs e)
{
    if (hexRadioButton.Checked)
    {
        var allowedValues = new[] 
        {
            'A', 'B', 'C', 'D', 'E', 'F',
            'a', 'b', 'c', 'd', 'e', 'f',
            '1', '2', '3', '4', '5', '6',
            '7', '8', '9', '0', '-'
        };

        var validString = new StringBuilder();
        var validChrIndex = 1;

        for (int i = 1; i <= inputBox.TextLength; i++)
        {
            var chr = inputBox.Text[i - 1];

            if (!allowedValues.Contains(chr)) continue;

            if (validChrIndex % 3 == 0)
            {
                if (chr != '-')
                {
                    validString.Append('-');
                    validChrIndex++;
                }
            }
            else if (chr == '-')
            {
                continue;
            }

            validString.Append(chr);
            validChrIndex++;
        }

        if (!validString.ToString().Equals(inputBox.Text))
        {
            inputBox.Text = validString.ToString();
            inputBox.SelectionStart = inputBox.TextLength;
        }
    }
}